(1) Field of the Invention
The present invention relates to lithium manganese-based composite oxides useful as positive electrode materials for next-generation, low-cost lithium-ion batteries, and a method for preparing such lithium manganese-based composite oxides.
(2) Description of the Related Art
A majority of secondary batteries presently mounted in portable equipment such as cellular phones, notebook computers, etc., in Japan are lithium-ion batteries. Such lithium-ion batteries are also expected to become practical as large batteries for use in electric vehicles, electric load leveling systems, etc., and are therefore increasing in importance.
A lithium-ion battery of today employs a lithium cobalt oxide (LiCoO2) as a typical positive electrode material, and a carbon material such as graphite as a negative electrode material.
In such a lithium-ion battery, the amount of lithium ions that are reversibly deintercalated (i.e., charged) and intercalated (i.e., discharged) into the positive electrode determines the battery capacity, and the voltages during deintercalation and intercalation determine the battery operating voltage. The positive electrode material LiCoO2 is hence an important material for battery constitution associated with the battery performance. The demand for lithium cobalt oxide, therefore, is expected to further grow with the increasing range of applications and increasing size of lithium-ion batteries.
Lithium cobalt oxide, however, contains a large amount of the rare metal, cobalt, thus being a cause of the high material costs of lithium-ion batteries. Further considering the fact that about 20% of cobalt resources are presently used in the battery industry, it seems to be difficult to meet the increasing demand only with positive electrode materials made of LiCoO2.
Lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4) and the like have so far been reported as lower-cost positive electrode materials that are less limited in the amounts of the natural sources, and some of which are in practical use as alternative materials. With lithium nickel oxide, however, the battery safety decreases as the deepness of charging increases, and with lithium manganese oxide, trivalent manganese dissolves into an electrolyte during charging/discharging at high temperatures (about 60° C.), causing significant deterioration in battery performance. Accordingly, the use of these materials as alternatives has not yet become prevalent. Another lithium manganese oxide, LiMnO2, has also been proposed as a positive electrode material. However, during charging/discharging, the structure of this material gradually changes to a spinel crystal structure, causing the shape of the charge/discharge curves to greatly change with charge/discharge cycles. LiMnO2, therefore, has also not come into practical use.
Moreover, the possibility of lithium ferrite (LiFeO2), containing iron that is more abundant in the amounts of natural resource, less toxic, and lower cost than manganese and nickel, for use as an electrode material has been explored. However, lithium ferrite obtained by a general method, i.e., mixing sources of iron and lithium, and firing the mixture at high temperatures, hardly becomes charged and discharged, and hence cannot be used as a positive electrode material for lithium-ion batteries.
On the other hand, LiFeO2 obtained by an ion exchange process has been reported to be capable of being charged and discharged (see Japanese Unexamined Patent Publications No. 1998-120421 and No. 1996-295518). This material, however, has an average discharge voltage of 2.5 V or less, which is remarkably lower than the value of LiCoO2 (about 3.7 V), and is hence difficult to use as a substitute for LiCoO2.
The present inventors have already found that a solid solution with the layered rock-salt type structure (Li1+x(FeyMn1-y)1−xO2, wherein 0<x<⅓ and 0<y<1; hereinafter referred to as “iron-containing Li2MnO3”) comprising lithium ferrite and lithium manganese oxide (Li2MnO3), which is the second most inexpensive and abundant material after iron, has an average discharge voltage of nearly 4 V, which is comparable to that provided with lithium cobalt oxide, according to charge/discharge tests at room temperature (see Japanese Unexamined Patent Publications No. 2002-68748 and 2002-121026).
Moreover, the inventors have found that a lithium-iron-manganese composite oxide that satisfies specific conditions exhibits increased capacity (150 mAh/g) and stable charge/discharge cycle characteristics compared to LiMn2O4 during cycling tests at high temperatures (see Japanese Unexamined Patent Publication No. 2005-154256).
As described above, various reports have been made on lithium manganese-based positive electrode materials that can substitute for lithium cobalt-based positive electrode materials; however, for further improvements in charge/discharge characteristics, optimization of the chemical composition, preparation conditions, etc., of positive electrode materials is desired.